1. Field of the Invention
The invention relates to deposition of a metal layer on a substrate. More particularly, the invention relates to anode configurations used in electroplating.
2. Description of the Background Art
Sub-quarter micron, multi-level metallization is an important technology for the next generation of ultra large scale integration (ULSI). Reliable formation of these interconnect features permits increased circuit density, improves acceptance of ULSI, and improves quality of individual processed wafers. As circuit densities increase, the widths of vias, contacts and other features, as well as the width of the dielectric materials between the features, decrease. However, the height of the dielectric layers remains substantially constant. Therefore, the aspect ratio for the features (i.e., their height or depth divided by their width) increases. Many traditional deposition processes, such as physical vapor deposition (PVD) and chemical vapor deposition (CVD), presently have difficulty providing uniform filling of features having aspect ratios greater than 4/1, and particularly greater than 10/1. Therefore, a great amount of ongoing effort is directed at the formation of void-free, nanometer-sized features having aspect ratios of 4/1, or higher.
Electroplating, previously limited in integrated circuit design to the fabrication of lines on circuit boards, is now being used to fill vias and contacts for the manufacture of IC interconnects. Metal electroplating, in general, can be achieved by a variety of techniques. One embodiment of an electroplating process involves initially depositing a barrier layer over the feature surfaces of the wafer, depositing a conductive metal seed layer over the barrier layer, and then depositing a conductive metal (such as copper) over the seed layer to fill the structure/feature. Finally, the deposited layers are planarized by, for example, chemical mechanical polishing (CMP), to define a conductive interconnect feature.
In electroplating, depositing of a metallic layer is accomplished by delivering electric power to the seed layer and then exposing the wafer-plating surface to an electrolytic solution containing the metal to be deposited. The subsequently deposited metal layer adheres to the seed layer to provide for uniform growth of the metal layer. A number of obstacles impair consistently reliable electroplating of metal onto wafers having nanometer-sized, high aspect ratio features. These obstacles include non-uniform power distribution and current density across the wafer plating surface to portions of the seed layer.
A system that electroplates a plating surface is depicted in FIG. 1. The device, known as a fountain plater 10, electroplates a metal on a surface 15 of a substrate 48 facing, and immersed in, electrolyte solution contained within the fountain plater. The electrolyte solution is filled to the lip 83 of the interior cavity 11 defined within the electrolyte cell 12. The fountain plater 10 includes an electrolyte cell 12 having a top opening 13, a removable substrate support 14 positioned above the top opening 13 to support a substrate in the electrolyte solution, and an anode 16 disposed near a bottom portion of the electrolyte cell 12 that is powered from the positive pole of a power supply 42. The electrolyte cell 12 is typically cylindrically-shaped to conform to the disk-shaped substrate 48 to be positioned therein. Disk-shaped contact ring 20 is configured to secure and support the substrate 48 in position during electroplating, and permits the electrolyte solution contained in the electrolyte cell 12 to contact the plating surface 15 of the substrate 48 while the latter is immersed in the electrolyte solution.
A negative pole of power supply 42 is selectively connected to each of a plurality of contacts 56 (only one is depicted in FIGS. 1, 2, and 4) which are typically mounted about the periphery of the substrate to provide multiple circuit pathways to the substrate, and thereby limit irregularities of the electrical field applied to the seed layer formed on the plating surface 15 of substrate 48. Typically, contacts 56 are formed from such conductive material such as tantalum (Ta), titanium (Ti), platinum (Pt), gold (Au), copper (Cu), or silver (Ag). Substrate 48 is positioned within an upper portion 79 of the cylindrical electrolyte cell 12, such that electrolyte flows along plating surface 15 of substrate 48 during operation of the fountain plater 10. Therefore, a negative charge applied from negative pole of power supply 42 via contact 56 to a seed layer deposited on plating surface 15 of substrate 48 in effect makes the substrate a cathode. The substrate 48 is electrically coupled to anode 16 by the electrolyte solution. The seed layer (not shown) formed on a cathode plating surface 15 of substrate 48 attracts positive ions carried by the electrolyte solution. The substrate 48 thus may be viewed as a work-piece being selectively electroplated.
A number of obstacles impair consistently reliable electroplating of copper onto substrates having nanometer-sized, high aspect ratio features. These obstacles limit the uniformity of power distribution and current density across the substrate plating surface needed to form a deposited metal layer having a substantially uniform thickness.
Electrolyte solution is supplied to electrolyte cell 12 via electrolyte input port 80 from electrolyte input supply 82. During normal operation, electrolyte solution overflows from an upper annular lip 83, formed on top of the electrolyte cell 12, into annular drain 85. The annular drain drains into electrolyte output port 86 which discharges to electrolyte output 88. Electrolyte output 88 is typically connected to the electrolyte input supply 82 via a regeneration element 87 that provides a closed loop for the electrolyte solution contained within the electrolyte cell, such that the electrolyte solution may be recirculated, maintained, and chemically refreshed. The motion associated with the recirculation of the electrolyte also assists in transporting the metallic ions from the anode 16 to the surface 15 of the substrate 48. In cases where the flow of the electrolyte solution through the anode does not conform to the general horizontal cross-sectional configuration of the electrolyte cell, the resultant electrolyte solution fluid flow through the electrolyte cell 12 can be irregular, non-axial, and even turbulent. Irregular and non-axial flows may produce eddies that lead to disruption of the metal deposition in the boundary layer adjacent to substrate 48. Such non-axial flow provides uneven distribution of ions across the selected portions of plating surface 15 of substrate 48. As a result, different regions of the electrolyte solution will have different concentrations of ions, which can lead to variations in plating rate on the substrate when such variation is present in the electrolyte solution that contacts the plating surface. This uneven deposition can result in an uneven depth of electroplated material. It is desired to provide an anode shape so that flow of the electrolyte solution is as uniform across the electrolyte cell 12 as possible. Therefore it is desired that the anode acts as a diffusion nozzle that provides a uniform flow across the cathode-substrate 48.
The anode 16 shape itself can lead to difficulties in the electroplating process. Irregular shapes of the anode 16, as well as irregular flows produced by the anode, are undesired. For example, if the anode 16 is located only on the left side of the fountain plater 10 in FIG. 1, then the left side of the surface 15 of the substrate will likely be coated more heavily than the right side of the surface 15. Irregularly shaped anodes 16 also affect the electromagnetic fields generated within the fountain plater 10, that can result in variation in the electrolyte solution contacting the plating surface of the substrate. Thus it would be desirable to produce an anode 16 having the same general shape as the cathode-substrate 48 to make the electomagnetic field across the cathode-substrate 48 more consistent and limit irregularities in the electrolyte solution contacting the plating surface, and thereby make the ions deposited on the substrate at a more even depth.
After a period of use, the anode 16 degrades due to exposure to the electrolyte solution especially where the electrolytic solution is directed at the anode 16 at a high velocity. This degradation can occur irregularly across the anode 16 resulting in an anode having an irregular depth. Such an irregular depth may result in uneven application of ions across the substrate-cathode 48 thus disrupting application of a conformal layer. It would be desirable to provide a system by which the anode 16 degradation is limited such that the original anode depth and shape is maintained. The anode should be easily replaced when worn or damaged, or the depth of the anode becomes irregular.
FIG. 1 shows a prior art hydrophilic membrane 89 that is fashioned as a bag to surround the anode 16. The material of the hydrophilic membrane 89 is selected to filter anode sludge passing from the anode 16 into the electrolyte solution, while permitting ions (i.e. copper) generated by anode 16 to pass from the anode 16 to the cathode 48. Hydrophilic membranes are well known in the art and will not be further detailed herein. Electrolyte solution that is input from the electrolyte input supply 82 via input port 80 is directed along a path shown by arrows 90 around the anode. The electrolyte solution carries metallic ions from the anode 16 to the cathode 48. The flow of anode ions actually extends up to the cathode.
The flow of electrolyte solution depicted by arrows 90 initially crosses anode hydrophilic membrane 89 at reference point 91. The electrolyte solution then interacts with anode 16 causing anode ions to be released at reference point 93 because of the electrolyte solution reacting with the anode. This reaction results in a release of a material from the anode called xe2x80x9canode sludgexe2x80x9d that is an unfortunate bi-product of the reaction. The anode sludge preferably remains contained within anode hydrophilic membrane 89, but may escape from the hydrophilic membrane under certain circumstances. If the anode sludge is released into the electrolyte solution, and is carried to contact the plating surface 15 of substrate-cathode 48 (especially if propelled at a high speed when it contacts the plating surface 15), then the impact of the anode sludge with the plating surface on the substrate 48 can damage the deposited layers. Therefore, it is desirable to provide a system that limits passage of the anode sludge into the electrolyte solution beyond or outside of the hydrophilic membrane.
The electrolyte solution carrying the anode ions crosses the anode hydrophilic membrane 89 again at a point indicated by reference character 95. The hydrophilic membrane 89 permitting this flow of the electrolyte solution is often referred to by those in the art as a flow-through membrane filter. A significant pressure-drop occurs each time the electrolyte solution passes through the hydrophilic membrane 89 depending partially on the filter size of the membrane filter selected. It would be desirable to reduce this pressure drop since some of the anode sludge can be forced through the membrane by the pressure drop. The pressure drop can further propel particles into contact with the substrate.
Therefore, there remains a need for an anode to be used in an electroplating apparatus that provides a substantially uniform ion flow and electric power distribution across a substrate. Such an anode configuration could be used to deposit a more reliable and consistent deposition layer on a substrate. There also remains a need to accomplish this uniform electric power distribution having a reduced pressure drop across the anode.
The invention generally provides an apparatus and associated method that deposits metal upon a substrate. The apparatus includes a perforated anode extending generally horizontally across the entire width of the metal deposition cell. Multiple perforations extend substantially vertically that provide a more uniform electrolyte flow across the width of the metal deposition cell. In one embodiment, a hydrophilic membrane limits particles that have eroded from the perforated anode from impinging upon the substrate. The membrane may extend across the entire width of the metal deposition cell. Alternatively, the membrane may extend across only those portions that lie directly above the perforated anode, therefore allowing electrolyte flow across the perforated anode without encountering a membrane.